Science is not licensed profession, and to be counted as a scientist one need not be a Doctor of Philosophy... But a scientist without a Ph.D. (or a medical degree) is like a lay brother in a Cistercian monastery. Generally he has to labor in the fields while others sing in the choir. If he goes into academic life, he can hope to become a professor only at the kind of college or university where faculty members are given neither time nor facilities for research... A young scientist with a bachelor's or a master's degree will probably have to spend his time working on problems or pieces of problems that are assigned to him by other people and that are of more practical than scientific interest. Wherever he works, the prospects are slight that he will be given much autonomy and freedom. Having a Ph. D. or its equivalent - a medical degree plus post-graduate training in research - has become in fact, if not in law, a requirement for full citizenship in the American scientific community.
To be successful as a scientist, it is important not only to have a Ph. D., but to have earned it at the right place. From the standpoint of rightness, American universities may be divided into three groups. The first is made up of those institutions to which the term "leading" may appropriately be applied. They include Chicago, Cal Tech, the University of California at Berkeley, Columbia, Harvard, Illinois, M.I.T. (=Massachusetts Institute of Technology), Michigan, Princeton, Stanford, Wisconsin, Yale, and perhaps two or three others. These are the universities whose professors get the biggest research grants, publish most scientific papers, serve on the most important government committees, win most of the scientific prizes, and are most likely to be acknowledged as leaders in their fields ... Ranking just below these twelve are universities like Minnesota and Indiana and U.C.L.A. (University of California at Los Angelos), where scientists and scholars of international renown are also to be found, but in such dense clusters as at Harvard or Berkeley ... This is not to say that first-rate scientists are to be found only at first-rate universities - or that are no second-rate people at Berkeley and M.I.T. But the brightest students, like the brightest professors, tend to be found at the leading universities.
Although possession of a Ph. D. is supposed to signify that a scientist has learned his trade as a researcher, it is now very common for young scientists to continue in a quasi-student status for a year or two after they get their doctorates. Older scientists as a rule are very happy to take on postdoctoral students. The postdoc, as he is sometimes called, is like an advanced graduate student in that he does research under the general direction of an older man. But he usually needs much less direction of an older man and he can therefore be much more helpful to an experienced scientist who is eager to see his work pushed forward as rapidly as possible... Postdoctoral trainees can have the further advantage of serving a professor as a middleman in his dealing with his graduate students.
For young scientists themselves, a year or two of postdoctoral study and research has many attractions. For some it is a chance to make up for what they didn't learn in graduate school. For scientists whose graduate training has been good, the chief advantage of doing postdoctoral research is that it gives them a couple of years in which they can put all their effort into research. A postdoctoral fellowship can also be a relatively tranquil interlude between the pressures and intellectual restrictions of life as a graduate student, and the competition and distractions of life as an assistant professor. Many scientists go abroad, not because the training they get will necessarily be better than they would get in the United States, but because a postdoctoral fellowship gives them a chance to travel - often for the first time in their lives.